1. Technical Field
The present invention relates generally to electrical probes and contacts for interconnecting integrated circuit (IC) devices and electronic components and systems, and specifically, to miniature high speed circuit probes and contacts for separably interconnecting IC devices such as bare chips, IC packages, and electronic modules, to other devices, motherboards, test systems, and alike.
2. Background Art
Contact probes are typically used in interposer sockets which provide separable interfaces for two oppositely disposed devices having corresponding input/output (I/O) patterns. A typical application is an electrical socket for connecting an IC device to the next level of electronic hardware or to a test unit. The contact probes are positioned and maintained in a required array by an insulating housing which has contact receiving cavities, disposed in a pattern corresponding to the array of I/O terminals in each of the mated devices. The contact probes typically rely on a compression spring to provide a resilient compliance and contact force to axially displaceable contact members or plungers. The plungers have contact portions adapted for connecting to a particular I/O terminal such as a solder ball of a ball grid array (BGA) device or a contact pad of a land grid array (LGA) device.
A typical 4-piece contact probe consists of a hollow barrel, a compression coil spring, and two plungers. The spring and the body portions of the plungers are retained and guided in the barrel, which is crimped at both ends. In order to reduce the contact probe resistance and inductance, the contact probes typically rely on the plungers randomly tilting (i.e., deviating from axial alignment with the barrel) and electrically shorting to the barrel, so that most of the current could bypass the spring and flow through the barrel. This significantly lowers the overall contact resistance of the probe and the parasitic electrical effects of the coil spring. The plunger-to-barrel contact depends on the spring bias, fit tolerances of the plunger and the barrel, contact surface topography, and plating uniformity on the inside of the barrel. The diametrical clearance between the plungers and the barrel must be precisely controlled to prevent an excessive plunger tilt.
While very reliable and durable conventional 4-piece probes can be produced in larger sizes, the desired probe characteristics is difficult to achieve in miniature probes, e.g., probes having an outside diameter of less than 1.0 mm and the length of less than 5 mm. In view of the machining tolerances and plating thickness variation on the inside of the barrel and on the plunger, the diametrical fit between the plunger and the barrel is difficult to tightly control. Since the plunger bodies which are guided in the bore of the barrel tend to be short, even a small diametrical clearance between the plunger and the bore can cause an excessive tilt of the plunger. The excessive plunger tilt can lead to a high localized wear and increased friction which may cause seizing of the plunger in the barrel, preventing the return of the plunger to the original extension, and thus causing a premature contact failure. Plating of high aspect ratio and small diameter barrels also creates challenges due to the difficulty of circulating the plating solution in a small bore. In addition, the base surface of the bore is often irregular and difficult to clean thoroughly prior to plating. The surface defects and plating irregularities can significantly affect the sliding friction and cause non-linear or erratic contact forces. The short probes also tend to have stiffer springs and less compliance. This can lead to a large variation in contact forces and cause damage to the I/O terminals (e.g. deformation or shearing of BGA solder balls) when the contact forces are excessive.
Numerous contact probes have been proposed to address the above issues by eliminating the barrel. Many of these probes utilize the spring as a conductive member. However, a coil spring acts as an electrical inductor at high frequencies and therefore presents electrical performance problems. Some designs aim to limit the parasitic effects of the spring by using springs with canted coils (closely wound coils which are at an oblique angle to the direction of compression) or by utilizing variable pitch springs with progressively closing (shorting) coils as the spring is being compressed. Other designs attempt to mitigate the adverse electrical effects of the spring by enabling a direct conductive contact between plungers. These 3-piece probes usually depend on a random biasing of the plungers against each other (e.g., due to spring load unbalance and plunger tilt). A direct resilient engagement between the plungers has also been proposed but it is particularly difficult to implement in miniature probes due to the difficulty of controlling the stiffness of the engagement means and the magnitude of the resilient engagement. A more effective direct plunger coupling mechanism, which overcomes these limitations is therefore highly desired.
The need for cost effective contact probes has also become more prominent in recent years. Of particular interest are 3-piece probes having stamped plungers, as illustrated by U.S. Pat. Nos. 8,033,872 B2 (2011) to Yang et al; 7,946,855 B2 (2011) to Osato; 7,862,391 B2 (2011) to Johnston et al; 7,841,864 B2 (2010) to Hsiao et al; 7,677,901 B1 (2010) to Suzuki et al; 7,256,593 B2 (2007) to Treibergs; 7,134,920 (2006) to Ju et al; and 7,025,602 B1 (2006) to Hwang. While stamping is prevalent in fabrication of electrical contacts and has the potential for providing cost effective high performance contact probes, stamped plungers also pose challenges and are particularly difficult to implement in small sizes, such as are required for mounting centers of about 1.0 mm. The challenges include effective utilization of space (fitting rectangular plunger sections inside the spring), maintaining coaxiality and pointing accuracy of the plungers (avoiding excessive tilt); avoiding sharp edge contact and parasitic friction between components, providing low and consistent contact forces when a direct resilient engagement between the plungers is employed, and providing an effective self-latching of the plungers to enable spring preload and one-piece contact probe handling. Consequently, there is a need for improved high performance contact probes which have cost effective stamped plungers, and which are suitable for use in test sockets and connectors having a close contact spacing, and a small connection height and thus low self-inductance.